Building a Kossel Mini? If you can't find those ball rails spec'ed for it, don't want to wait until the Kossel Pro kickstarter ends, and want to save over $50, this part gives you another option.
This is a Cerberus Pup-style carriage for Kossel Mini. It provides smooth motion with low cost (~$70 for 3) on OpenBeam, using Graber Dual 623 V rollers. It supports an unmodified Kossel ball joint adapter (ball joint adapter) since the 20x20 m3 grid matches the spacing on the ball rails specified for the Kossel Mini. Just drop it in, and besides this part, nothing needs to change on your Kossel Mini build.
If the ball rails sold by OpenBeamUSA are:
- not in stock,
- too expensive ($150), or
- not long enough (only 400mm limits Kossel size to slightly over 600mm)
... then this part may be for you.
Requires 3x Graber Dual 623 V rollers per carriage, which are $7.45 apiece.
Note the non-commercial CC license - a requirement, since this part derives from Chillance's original integrated version.
Required Parts (per carriage):
- 7x m3 nuts
- 3x m3 nyloc nuts - regular m3 will do too, in a pinch.
- 1x m3x16mm screw
- 5x m3x25mm screw
- 1x m3x40mm screw (for tensioner)
- 1x m3 washer
- 3x Graber Dual 623 V rollers
I purchased all of the fasteners from MrMetric.com, and splurged on stainless socket head cap screws.
The list is fairly long, but aside from printing, getting these rolling won't take much time.
- Print at low-to-moderate infill. 30% with 3 perimeters is plenty rigid for me, but you could always go higher. I've done 0.4mm layers with a 0.5mm nozzle with good results.
- Drill out the 3 holes in the m3 grid with an m3 bit; the part has a thin bridge at each hole to avoid having to print overhangs around these holes. Make sure to clear out the drilled swarf with an exacto knife; if you don't the nut you press in next won't sit flat.
- Drill out the long hole for the tensioning screw with an m3 bit.
- Drill out the holes for the rollers with a slightly-smaller-than-m3 bit.
- Press 3x m3 nuts into the rear of the m3 20x20 grid and 1x m3 nut into the hole for the long tensioning screw. My technique: take one of the roller screws, screw on a nut, and then tap it with a hammer to press the nut into the trap.
- Insert 3 m3x25 screws into the counterbored holes at the edge to later attach the rollers. I suggest using a hammer to get the screw started in the tight hole, then having the screw form a thread from there by tightening the allen wrench. This ensures that there will be no play in the rollers when you tension them using the tensioning screw. It also prevents the roller screws from turning, which comes in handy when tightening nuts on them, next.
- Add a single nut to each roller screw to bridge the gaps from the plastic part edges to the roller edges, then lock it down. Must have enough spacing so that the printed part never touches the OpenBeam; add extra washers if necessary, but I didn't need any.
- Add 3x 623 Dual V-Wheels.
- Add 3x m3 nyloc nuts to secure the wheels and carefully tighten until there is a bit of resistance, but not so much that the roller rolls with resistance. No need to go super-tight and damage the bearing, but you do want to eliminate any slop here. Tighten, test, and then adjust for each roller.
- Add a washer to the 40mm long m3 tensioning screw, then screw it in until the washer just touches the plastic. The washer helps keep the screw threads just within the part envelope, so you won't get scratched if you grab the carriage.
- Add Kossel Mini ball joint adapter or any other one with a 20x20 grid.
- Add a m3x16 screw for the bottom corner of the ball joint adapter, plus two m3x25 screws for the top two holes.
- Slide completed assembly onto an OpenBeam rail.
- Adjust tensioning screw with fingertips, then an allen wrench, until sliding the roller feels right; there should be zero backlash. In other words, it shouldn't have perceptible motion when you move the part. There should be minimal resistance, but no motion when you twist the carriage.
Deltabot Google group post with more info and pictures.
OpenSCAD source and latest mods are available on the GitHub repo.
Steve Graber made the original design for Cerberus-Pup. Daniel Akesson converted the original into OpenSCAD. I tweaked the OpenSCAD to:
- be compatible with linear rails (thicker, w/20x20 m3 mounting grid)
- remove ball joint screw mounts
- fix the size (too loose)
For reference, the Kossel Pro and Mini ball rails from OpenBeamUSA are equivalent to [HIWIN Model MGN 12H](http://hiwin.com/html/extras/mgn0 -c_mgn-h.html).
Weight of plastic part only printed at 30% infill, 0.4 layers, 0.5 nozzle, 3 perimeters, in PLA: 16g
Weight of assembled carriage, including rollers, screws, and nuts (but no ball joint adapter or screws for it): 36g